SRP stands for Single Responsibility Principle. This states that a class should have only one reason to change. If a class is doing too much (i.e. hitting the DB, writing files, doing business logic, calling a web service, etc, it’s violating SRP. It will be very difficult to change later should we need to change it, and it will likely be prone to have more defects.

SRP applies to the methods or in a class also. The two are inseparable. A class, or object, is the combination of its data and its behavior.

For more details read this document: Single Responsibility Principle
http://www.objectmentor.com/resources/articles/srp.pdf